1. Field of the Invention
The present invention relates to a liquid droplet ejection apparatus, and more particularly, to a liquid droplet ejection apparatus in which droplets are ejected from a plurality of ejection ports to form dots on a recording medium.
2. Description of the Related Art
An inkjet type of image forming apparatus has a print head formed with a plurality of nozzles (ejection ports), and forms images on a recording medium by ejecting ink droplets from nozzles onto a recording medium, while conveying the print head and the recording medium relatively with respect to each other. The printing method in an image forming apparatus of this kind may be a shuttle method which performs recording by scanning the recording medium in the breadthways direction thereof with a short serial head, and a line method which uses a line head in which nozzles are arranged so as to correspond to the full width of the recording medium.
In an image forming apparatus of this kind, during printing, the nozzles are always filled with ink, in such a manner that printing is carried out immediately when a printing instruction is issued. Therefore, if nozzles remain in a state in which ink droplets are not ejected for a prescribed time period or longer, then the ink in the vicinity of the nozzles increases in viscosity, and even if a normal ink ejection signal is subsequently applied, there may be variation in the dot size or dot landing positions, or the nozzles may become blocked, in such a manner that it becomes impossible to eject ink droplets (hereinafter, these situations are referred to generally as “ejection defects”). Therefore, in a print head, maintenance operations are carried out at prescribed time intervals in order forcibly to eject or suction ink of increased viscosity which causes ejection defects. Maintenance operations of this kind reduce the printing speed and cause wasteful consumption of the ink.
In order to resolve problems of this kind, Japanese Patent Application Publication No. 2002-240257 discloses a droplet ejection control method which performs a print operation by detecting nozzles that have not been used for a prescribed period of time, and changes the scanning data (dot data) and paper conveyance amount in such a manner that these nozzles are used. The droplet ejection control method according to Japanese Patent Application Publication No. 2002-240257 is described with reference to an example shown in FIG. 14. In FIG. 14, a print operation is performed by moving a print head 150 in a scanning direction perpendicular to the paper conveyance direction, while conveying a recording medium in the paper conveyance direction. Each of the dots on the recording medium indicates scan data which has been converted from print data (image data). The print head 150 has five nozzles N1, N2, N3, N4 and N5, and is able to print five lines by means of a single printing operation. When the printing of one scan has completed, the recording medium is conveyed in the paper conveyance direction through a distance corresponding to five lines. By repeating the scanning movement of the print head 150 and the conveyance of the recording medium in this fashion, a normal printing operation is carried out. Here, it is supposed that, at the end of the printing onto region A in the initial scanning, nozzle N3 is determined to be an unused nozzle that has not been used once during a certain prescribed time period. Normally, in the next scan, the nozzle to print line B1 would be nozzle N1, and the nozzle to print line B2 would be nozzle N2, but according to this droplet ejection control method, the next scanning data is rewritten in such a manner that lines B1 to B4 are printed respectively by nozzles N2 to N5 so that the nozzle N3 is used. Here, the recording medium is conveyed through a distance of four lines, which is reduced by one line from the normal paper conveyance amount (five lines).
However, in the droplet ejection control method disclosed in Japanese Patent Application Publication No. 2002-240257, if an unused nozzle which has not been used for a prescribed period of time is detected, then the scanning data and the paper conveyance amount are changed in such a manner that the unused nozzle is used, regardless of the effects on the image that might occur if the unused nozzle is suffering an ejection defect and droplets cannot be ejected correctly from the unused nozzle in order to form the dots that are supposed to be formed.
For example, in the example shown in FIG. 14, there is no scan data in the lines A3, B3 and C3, which are originally to have been printed by the nozzle N3 that has been determined to be the unused nozzle, and therefore, even if the nozzle N3 is suffering an ejection defect, this has absolutely no effect on the image. However, in the droplet ejection control method in the related art, the scan data is changed in order to use the nozzle N3 and the paper conveyance amount is reduced accordingly, thereby leading to a decline in the printing speed.